Abstract On 2024 October 5, BL Lacertae (2200 + 420) experienced one of its brightest gamma-ray flares. We conducted simultaneous follow-up observations in the u, v, g, r, i, and z bands from 2024 October 17 to November 21 using the Mephisto telescope and its two 50 cm twin auxiliary photometric telescopes of Yunnan University. Intraday variability (IDV) was detected in the g, r, i, and z bands. The IDV duty cycle increased with observing frequency across these bands. The shortest variability time-scale, derived from auto-correlation analysis, constrains the upper limit of the black hole mass to be M• ≲ 108.29 M⊙ assuming a Kerr black hole, and M• ≲ 108.77 M⊙ assuming a Schwarzschild black hole. The emission region responsible for the observed variability has a size of R ≤ 3.51 × 1014 cm and is located at a distance of RH ≤ 2.83 × 1015 cm from the central supermassive black hole. This distance is approximately three orders of magnitude smaller than the typical radius of the broad-line region, indicating that the emission region lies well within it. A general bluer-when-brighter (BWB) trend was detected on intraday time-scales, suggesting that shock-accelerated relativistic electrons enhance the high-energy particle population, leading to spectral hardening. A potential quasi-periodic oscillation (QPO) with a period of ~100.77 minutes was detected with 99.99 per cent confidence, consistent with predictions from the magnetic reconnection model. These observed optical intraday variabilities and colour variations of BL Lacertae can be well explained by the turbulent jet model.
Chang et al. (Wed,) studied this question.